Field of the Invention
The present invention relates to a sheet conveyance apparatus for conveying sheets, and an image forming apparatus equipped with the same.
Description of the Related Art
Hitherto, image forming apparatuses, such as printers, are equipped with a sheet feeding apparatus, having sheets for recording images stacked on a tray and capable of separating and feeding the sheets one by one. Japanese Patent Application Laid-Open Publication No. 2003-118865 discloses a sheet feeding apparatus having a guide member for guiding sheets toward a separation nip for separating and feeding sheets one by one. The guide member is constituted movably when a bundle of sheets are fed.
Now, a conventional sheet feeding apparatus will be described with reference to FIGS. 18 and 19. A sheet feeding apparatus 301 is equipped with a pickup roller 310, a feed roller 353 and a retard roller 364, wherein a separation nip N is formed by having the retard roller 364 pressed against the feed roller 353 with a given pressure. A rotational driving force in a direction of arrow J1 is transmitted from a drive source not shown to the feed roller 353. A rotational driving force in a direction opposite to a sheet feeding direction (direction of arrow J3 of FIG. 19B) is transmitted via a torque limiter not shown to the retard roller 364.
Further, the sheet feeding apparatus 301 is equipped with a nip guide 363 preventing the sheet S from being caught between a sheet cassette 300 and the separation nip N and jammed. The nip guide 363 is supported rotatably on a rotating shaft 363a, and biased toward a direction approaching the feed roller 353 (direction of arrow J2) by a tension spring 366.
As shown in FIG. 18, a guide surface 363b of the nip guide 363 is positioned by a stopper 365 so as to form a given angle θ (0<θ<90 degrees) with respect to a sheet feed direction H of the sheet S stacked on an intermediate plate 351 pushed up by a turning arm 356. When power is transmitted to the pickup roller 310, the feed roller 353 and the retard roller 364, and a single sheet S is fed by the pickup roller 310, the sheet S is guided by the nip guide 363 to a separation nip N, as shown in FIG. 19A. In other words, when a single sheet S is fed, hardly any load is applied from the sheet S to the guide surface 363b of the nip guide 363, so that the nip guide 363 remains abutted against the stopper 365 without rotating.
On the other hand, when multiple sheets S forming a bundle is sent out from the sheet cassette 300, a large amount of load is applied to the nip guide 363 from the bundle of sheets S. Thereby, as shown in FIG. 19B, the nip guide 363 rotates in a direction separating from the feed roller 353 (direction of arrow d). Even when only a single sheet S is fed by the pickup roller 310, if the sheet is a cardboard having a high stiffness, for example, a large load is applied on the nip guide 363, and the nip guide 363 rotates in a direction of arrow J4. When the nip guide 363 rotates in the direction of arrow J4, the given angle θ formed by the guide surface 363b and the bundle of sheets S is increased, so the sheets S are sorted while being guided to the separation nip N.
Meanwhile, Japanese Patent Application Laid-Open Publication No. S63-225043 teaches a sheet feeding apparatus equipped with a separating roller frame holding a separating roller (retard roller) in a swingable manner, and a feed-in guide plate supported rotatably on a separating roller frame and guiding the sheet to the separation nip. The feed-in guide plate is designed to rotate downward to prevent jamming of sheets when a large number of sheets are fed between the feed-in guide plate and a feed roller forming the separation nip together with the separating roller.
As described above, according to the sheet feeding apparatus disclosed in Japanese Patent Application Laid-Open Publication No. 2003-118865, the given angle θ between the nip guide 363 and the sheet S varies depending on the level of load applied to the nip guide 363 from the sheet S. For example, if a card board is fed from the sheet cassette 300, the nip guide 363 is pressed by the card board and rotates downward easily. Then, when the nip guide 363 is pressed by the sheet S and is migrated in a direction of arrow O, the abutting angle in which the front end of the sheet S abuts against the peripheral surface of the retard roller 364 is increased.
In order to correspond to the downsizing of recent printers, there is a tendency to minimize the outer diameter of the retard roller 364. In that case, the given angle θ between the nip guide 363 and the sheet S is changed, and the abutting angle between the front end of the sheet S and the peripheral surface of the retard roller 364 is increased significantly. Generally, the peripheral surface of the retard roller 364 is formed of a material having a high friction coefficient, so that if the front end of the sheet S collides against the retard roller 364 with a great abutting angle, the front end of the sheet S may be damaged greatly, or jamming of the sheet S may occur.
In the sheet feeding apparatus disclosed in Japanese Patent Application Laid-Open Publication No. S63-225043, the range of rotation of the feed-in guide plate is regulated by an upper limit stopper and a lower limit stopper disposed on a separating roller frame. The space between the upper limit stopper and the lower limit stopper is small, and the range of rotation of the feed-in guide plate is narrow. When the feed-in guide plate is pressed by a large number of sheets and rotates downward, the feed-in guide plate abuts against the upper limit stopper, but when the feed-in guide plate rotates further downward, the separating roller frame rotates downward together with the feed-in guide plate. Then, the separating roller held in the separating roller frame will be separated from the feed roller, and there is fear that the large number of sheets cannot be separated one by one and overlapped feeding may occur.